Method for backwashing filters

ABSTRACT

A vertical vessel has a filter bed disposed in its lower portion for normal filtering of solid particulates from contaminated liquid passed down through the bed. Vertical passages are formed to extend below the upper surface of the filter bed with a layer of smooth-surfaced bodies arranged to reduce the cross-sectional area of the vessel to impart desired velocity to the particulates in the liquid. Conduits are provided to flow liquid and gas up through the bed to backwash particulates from the bed into a collection above the bed which is removed by a siphon in a batch operation.

Elite States Patent 72] Inventor Isaac Paul Mail Tulsa, Okla.

[21] Appl. No. 40,249

[22] Filed May 25, 1970 [45] Patented Jan. 4,1972

[73] Assignee Combustion Engineering, Inc.

New York, N.Y.

Contlnuation-in-part of application Ser. No. 31,935, Apr. 27, 1970, which is a division 01 application Ser. No. 737,721, June 17, 1968, now abandoned which is a continuation-in-part of application Ser. No. 857,266, Aug. 8, 1969, now Patent No. 3,552,573. This application May 25, 1970, Ser. No. 40,249

[54] METHOD FOR BACKWASHING FILTERS Primary Examiner-Samih N. Zahama Att0rneyArthur L. Wade ABSTRACT: A vertical vessel has a filter bed disposed in its lower portion for normal filtering'of solid particulates from contaminated liquid passed down through the bed. Vertical passages are formed to extend below the upper surface of the filter bed with a layer of smooth-surfaced bodies arranged to reduce the cross-sectional area of the vessel to impart desired velocity to the particulates in the liquid. Conduits are provided to flow liquid and gas up through the bed to backwash particulates from the bed into a collection above the bed which is removed by a siphon in a batch operation.

INLET/ WATER TO BE FILTERED 4- INLET 0R VENT/GAS F/LTERED WATER msmznm 4312 3631.933

INLET/ WATER TO BE F/LTERED INLET 0R VENT/GAS BACK WASH 1 OUTLET OUTLET/ F ILTERED WATER l7 INVENTOR.

/S P U MAIL INLE 776145 AAC A L /5 ATTORNEY METHOD FOR BACKWASI'IING FILTERS BACKGROUND OF THE INVENTION 1. Field of the Invention This application is a continuation-in-part of Ser. No. 31,935, filed Apr. 27, 1970. Ser. No. 31,935 is a division of Ser. No. 737,721, filed June 17, 1968 and now abandoned. A continuation-in-part of Ser. No. 737,721 was filed Aug. 8, 1969, and is Ser. No. 857,266, now US. Pat. No. 3,552,573.

The present invention relates generally to filtering apparatus and more particularly is directed to improvements in a backwashing method for a filtering apparatus of the downflow type; that is, in improvements in a backwashing method by a novel manner of batching in the individual backwash cycles in filtering apparatus in which the liquid to be filtered is passed downwardly through a filter bed of granular solids.

2. Description of the Prior Art In the filtering apparatus presently is use, the liquid to be filtered and backwash liquids have been passed both upwardly as well as downwardly through a filter bed of granular solids. There are advantages and disadvantages associated with each flow pattern.

It is presently accepted that with a downflow pattern, the particulates are removed from the liquid near the upper surface of the bed and collect in a cake" which is supported by the lower bed. This cake quickly increases the pressure drop in the downflowing liquid which is a common index for limiting the filter cycle.

It is also presently accepted to backwash a filter bed within a vessel by continuously passing washing liquid through the bed in a direction opposite to the direction of filtering flow. The velocity of the liquid must be great enough to expand the bed so as to dislodge the filtered particulates from the bed material. The total quantity of liquid required to return the bed to serviceable cleanliness is large.

SUMMARY OF THE INVENTION A principal object of the invention is to direct liquid to be filtered downwardly into a bed of granular material and impart enough kinetic energy to the particulates for them to overcome the frictional forces imposed upon them by the surface of the granular material. The particulates are thus caused to penetrate into the bed a desired distance.

Another object is to jet particulates downward into a bed of granular material so the particulates will penetrate the bed a desired depth and then backwash the bed in an improved manner with fluids directed upward through the bed.

Another object is to backwash the bed of granular material in a batch operation and employ gas bubbles to agitate the bed material to aid in dislodgment of particulates deposited in the bed.

The present invention contemplates a vessel having a bed of solid granular material supported therein and arranged to receive a downward flow of liquid from which it is desired to remove particulates. A structure is mounted near the upper portion of the bed which will form multiple vertical passages extended below the upper surface of the bed and whose total cross-sectional area is substantially less than the cross-sectional area of the vessel. This reduction of the cross-sectional area results in the downflowing liquid increasing in velocity as it flows through the multiple passages. The increased velocity imparts to the particulates sufficient kinetic energy to cause them to penetrate into the bed a predetennined depth.

The invention further contemplates a unique method of cleaning the bed of particulates removed from filtered liquid. The bed is physically arranged spaced from the upper end of its vessel. Backwash liquid is then passed upwardly through the bed to partially fill the space above the bed. Gas bubbles are concurrently passed up through the bed with the liquid to facilitate agitation of the bed which will dislodge particulates. After the space is partially filled with backwash liquid, the supply of gas is discontinued. The flow of backwash liquid is continued to purge the bubbles from the bed. Any dislodged bed material settles back into the bed during this period. The

liquid above the bed, with the particulates entrained therein, is then removed while continuing the flow of backwash liquid up through the bed. The cycle may be repeated in a batch operation until the particulate contamination of the bed has been satisfactorily reduced.

Other objects, advantages and features of this invention will become apparent to one skilled in the art upon consideration of the written specification, appended claims, and attached drawing, wherein;

FIG. I is a sectioned schematic elevation of a filtering apparatus including the present invention; and

FIG. 2 is a plan view of FIG. I taken along the line 2-2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Basic Structural Arrangement FIG. 1 discloses the subject invention embodied in the filtering apparatus generally identified by the numeral 1. All of the structure of the apparatus centers about the vertically extended vessel 2. The filtering apparatus disclosed herein is of the downflow type. The raw influent is introduced into vessel 2 through conduit 3 for downward passage through filter bed 4, while the effluent filtrate is withdrawn from the base of vessel 2 through conduit 5 controlled by a valve 19. Also within the vessel 2 is a layer of smooth-surfaced bodies 6 supported by a wire mesh screen 7 and a perforated support structure 8 for bed 4.

The contaminated, or raw, influent entering vessel 2 through conduit 3 is given a predetermined flow rate. Since the velocity of the influent through the filtering medium is the critical variable in the design of the apparatus, the total crosssectional area of the flow paths to the top surface of the filter bed 4 between bodies 6 (and hence the diameter of the vessel) is sized dependent upon this fiow rate.

TESTS OF A CONVENTIONAL SYSTEM Tests were conducted using a filter medium of granular solids now commonly employed in treating water associated with oil production and water produced from water supply wells for injection purposes in secondary recovery operations. The particulate material and size was that also found in such produced waters.

It was observed during this testing that flow velocity normally developed allowed to particulates to form a filter cake of low permeability to the influent on the top surface of the filter medium. The filter medium then became merely a support for this filter cake, and the cake, in efiect, became the filter medium. These velocities are conventionally less than 2 ft./min. and were ordinarily 0.3 to 0.5 ft./min.

This continuing buildup of a filter cake on top of the medium results in an increasing pressure differential across the filtering apparatus, a decreasing throughput, and the need for frequent cleaning of the filter.

Basic Filtering Operation The influent moves downwardly through the vessel 2 to the layer of smooth-surfaced bodies 6 supported by wire mesh screen 7. Screen 7 extends across the cross-sectional area of vessel 2 and is situated below the top surface of the filter bed 4.

The layer of bodies 6 forms a restriction to the flow of fluid through the full diameter of vessel 2. The influent must flow through the paths formed by the spaces between the bodies. These spaces form restrictive paths to the flow and, in efi'ect, act as jets or nozzles. This jetting action increases the flow velocity of the influent at the surface of the filter medium 4 into a desired range.

Experiment established that the lower of the bodies 6 must be at least partially below the upper surface of the filter bed as shown in FIG. I. From another view, the upper surface must extend up into the restrictive paths formed by the spaces between the bodies 6 which act as jets or nozzles. If the bed surface is at, or below, the lower surface formed by bodies 6, the objectionable filter cake will build up on the top of the filter bed with the disadvantages found in the prior art.

With the bed 4 and bodies 6 in correct positional relation, the desired range of flow rate of the influent imparts increased kinetic energy to the particulates in the liquid and drives the particulates below the upper boundary of the filter medium. At the surface of the filter bed, the bed does become saturated with particulate material, but other particulates will nevertheless be carried into lower unsaturated portions of the filter bed with their increased kinetic energy. The formation of a low permeability filter cake on the surface of the filter medium is thus obviated This filtration phenomenon has been analogized to adsorbent system mechanics. The zone through which any particulate will travel before being stopped is comparable to the mass transfer zone of an adsorbent system. Bed depths are designed to accommodate and exceed the length of the mass transfer zones created by the particular flow velocity anticipated.

The bodies 6 function to reduce the cross-sectional area of the conduit (vessel 2) at the upstream surface boundary of the filter medium. This reduction in area increases the flow velocity of the contaminated influent at this boundary and causes the influent to move across the boundary region and into the medium.

Also, although spherical bodies 6 are disclosed, it is apparent that other means are available to impart the desired velocity gain to the influent at the upper boundary of the filter bed. Although there are many forms this structure could take to carry out the objectives of the invention, they should all produce a jetting effect which will drive the particulates down into the bed and permit the ready return of displaced bed particles. Further, the upper boundary of the bed must extend up into the passages formed by the structure producing the jetting effect to obviate formation of a filter cake.

It is obvious that the smooth-surface bodies 6 could take a variety of shapes and materials. For example, they could be oblate or elongate spheroidal, or irregularly rounded (as smooth river stones). They could be formed from ceramic, glass, plastic, or metallic material. It is only imperative that they reduce the cross-sectional area of vessel 2 sufficiently to impart adequate velocity to the flow, without themselves being so small or so rough surfaced as to initiate the filtering action.

It is noted that flow velocities above a certain value will expand or disrupt the granular or sand-type filter medium depicted in FIG. 1. Velocities in this range exert sufficient pressure on the filter bed surface to produce a coning, channeling, or drilling effect which will reduce the effectiveness of the filter bed. Velocities on the order of 7 ft./min. (the high range of those tested) did not expand or disrupt the filter medium under test, however.

Backwashing After the bed 4 is saturated with particulates by the novel arrangement disclosed above, the filtered material entrained in the bed must be periodically removed. It has been common practice to simply introduce a liquid beneath the bed and flow the liquid upward with a velocity sufficient to dislodge the particulates. Gas has also been introduced up through the bed to agitate it and thereby loosen the bond between the particulates and the bed material. Some expansion of the bed takes place and the velocity must be carefully controlled to avoid losing a significant portion of the bed material entrained in the backwash liquid. The present invention includes a method which provides the upflowing backwash liquid, with agitating gas, but without significant loss of bed material. Further, the quantity of backwash liquid required is low and the rate of flow of this liquid can be kept quite low.

Fundamentally, the method of the present invention includes employing a number of backwash cycles. A predetermined amount of liquid is passed up through the bed 4 to a predetermined level above the bed. Gas is used concurrently with the backwash liquid and/or in sequence with it. The predetermined amount of backwash liquid above the bed with particulate material swept from the bed below, is then removed. Another amount of backwash liquid is again employed in the same manner and removed. This cycle is repeated enough times to reduce the bed contamination to the level desired for filtering service.

More specifically, referring to FIG. 1, when bed 4 is saturated by particulates, the flow of liquid to be filtered is terminated. A valve 9 is indicated in conduit 3 for this control of flow into vessel 2.

The next step is to evacuate the upper portion of vessel 2 of liquid down to the bed 4. Horizontal index line A indicates this level near the top of the bed 4-bodies 6. Outlet conduit 10 is shown as available for this purpose, valve 11 in the conduit providing the control.

Valve 11 is opened and the liquid above the bed will flow out conduit 10 if the conduit is filled with liquid. However, if necessary to prime this siphon system, gas can be applied to the upper portion of vessel 2 through conduit 12. Valve 13 is provided in conduit 12 for control. The pressure of the gas will force the liquid up conduit 10 and through valve 11. Conduit 12 is also provided as a vent line for the air in the space above the bed which is displaced by the entry of backwash fluids.

A backwash liquid, from a source not shown, is next conducted into the lower portion of vessel 2. Conduit 5 could be used for this purpose, but separate conduit 14 is shown to give clarity to the disclosure. Valve 15 is indicated in conduit 14 for this control.

Valve 11 in conduit 10 is closed, so the backwash liquid travels up through the bed 4 to dislodge particulate material from the bed. Gas is also flowed up bed 4. This gas is conducted into vessel 2 through conduit 16. This conduit is disclosed as arranged to introduce the gas into the backwash liquid of conduit 14 to back up the concept of flowing the gas concurrently with the backwash liquid. Valve 17 is provided in conduit 16 for control. The precise arrangement of these conduits and vessel 2is not disclosed to limit the invention. The concept is to flow an amount of backwash liquid and gas up through bed 4 to raise the liquid level from level A to level B. Gas is to be used to supplement the agitating effect of the liquid. The gas may be used concurrent with the liquid or in sequence with it. After the agitation has driven a maximum amount of particulates into the liquid between levels A and B, additional backwash liquid is introduced through conduit 14 to flush the gas from the bed.

The higher liquid level is indicated at C, substantially the remaining portion of the vessel 2. The goal is to provide enough backwash liquid to flush the gas from bed 4 so that any bed material carried up into this vessel space can settle back to bed 4 by gravity. Ifthe gas were not positively flushed from the bed, it would continue to break out and tend to retard settling of the bed material.

With level C reached by the backwash liquid in vessel 2, the fluids in the space above the bed are siphoned through conduits 18, 10, while continuing the flow of backwash liquid. Gravity causes the bed material to settle against this flow; the relatively fine material will slide over the smooth surface of bodies 6 and into bed 4. The particulate material dislodged into the backwash liquid and entrained in this liquid in the space above the filter bed is removed at a substantially greater rate than the continuing inflow of backwash liquid. When removal is completed and level A is attained, the siphon 10, 18 is closed by valve 11, and another cycle of backwashing is begun by introduction of another batch of backwash liquid and gas into the space above the bed.

Valve 11 is opened and the contaminated backwash water is drawn from the vessel 2 at level A through the lower end'of conduit section 18 which is the internal end of conduit 10 inserted into vessel 2. Conduit 10 is formed with section 18 extending downwardly to return any bed material to the vessel and prevent buildup in conduit 10 which might obstruct the outlet. This run of conduit upward will give a measurable opportunity for gravity to become effective during the settling period following agitation.

This method of running a batch of backwash liquid up through bed 4 can be repeated as often as required to clean bed 4. Obviously the valves controlling the conduits can be automated to carry out this method of cleaning. The positive, batch removal of backwash liquid and gas requires less total liquid than would the flow of liquid alone to first purge the bed and then sweep the vessel above the bed clean.

From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects herein set forth, together with other advantages which are obvious and which are inherent to the method.

It will also be seen that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed is:

l. A method of backwashing a filter bed of granular solids in a batching operation in each backwash cycle and in the order of steps recited, including, providing a space above the filter bed, shutting off the flow of fluids to be filtered, I

opening a gas inlet to cause displacement of the water in the space above the filter bed through a siphon,

shutting off the siphon when substantially all the water has been siphoned from the space above the filter bed,

flowing water and gas up through the filter bed to disengage particulates from the bed by dislodging the bed and venting the air displaced from the space above the bed by the entry of water and gas,

shutting off the flow of gas through the bed when the water in the space above the bed has attained a first predetermined level to allow the bed to settle during a continuing upward flow of water, and

opening the siphon when the water above the filter bed has attained a second predetermined level due to the continued upward flow of water during the period of settling of the bed and drawing off the water and any gas still mixed therewith and particulates backwashed from the bed through the siphon, while still continuing the upward flow of backwash water at a substantially lesser rate than the siphoning rate throughout the siphoning-off process to allow the most efficient reconsolidation of the filter bed and removal of particulates entrained in the backwash fluids above the filter bed.

2. The method of claim 1, including,

repeating the cycle of claim 1 until the bed is clean enough for filtering service.

3. The method of claim I, in which,

the gas displacing the liquid above the bed through the siphon is air at atmospheric pressure.

4. The method of claim 1, in which,

the gas displacing the liquid above the bed through the siphon is under pressure. 

2. The method of claim 1, including, repeating the cycle of claim 1 until the bed is clean enough for filtering service.
 3. The method of claim 1, in which, the gas displacing the liquid above the bed through the siphon is air at atmospheric pressure.
 4. The method of claim 1, in which, the gas displacing the liquid above the bed through the siphon is under pressure. 